The present invention relates to a plant designed for use in digging and shoring up excavations during tunnelling works.
In recent times, a number of different techniques have been employed in the construction of tunnels, for road or rail, for utilities such as sewers or underground canals, and for the routing of water courses generally.
One such technique involves the use of mechanical shields to carry the excavation forward while installing pre-cast lining sections (tubular or segmental) as digging proceeds. These shields can be driven forward by hydraulic jacks from outside, using the pre-cast tunnel linings already installed as intermediate elements in a growing chain.
The distances obtainable using this expedient are not great, however, given that friction increases progressively with length, and impossible demands are put on the strength of the intermediate linings as greater and greater thrust is applied.
The shield can also be driven forward utilizing the previously installed section of the tunnel lining (pre-cast or cast-in-situ) as a fixed bearing. In this instance, however, limitations are imposed on forward progress by the capability of the lining to withstand the thrust of the hydraulic jacks; more exactly, on encountering a degree of lateral friction or resistance to penetration greater than the mechanical strength of the lining, the shield obviously can no longer operate, inasmuch as the force required to produce increased thrust would destroy the lining.
This drawback is partly overcome in the majority of cases by installing intermediate stations as shown in German patent application n 3 032 856, against which to exert the necessary thrust, though the reaction force from such stations is transmitted just the same either to adjacent sections that are not designed to withstand high pressures, and thus will be in danger of breaking up, or to a lining cast in-situ, which becomes subject to considerable stresses.
In addition, increased friction must be overcome in driving forward where the bore has to follow a gentle bend; in this particular situation moreover, the direction of thrust no longer coincides with the bore axis, and there is no means of altering the direction except by way of the jacks, which are located remotely from the section that is required to change course.
An apparatus for tunnelling through soft stratum illustrated in UK patent application n 2 180 867 comprises a body portion having a plurality of cylindrical members connected together, a head portion connected to one of the cylindrical members at one of its ends, a waterproof frame disposed between the head portion and cylindrical member, a plurality of pneumatic cylinders disposed between each two of the cylindrical members and a plurality of grouting pipes mounted at the tail end of the body portion. With this apparatus the reaction force is transmitted to cement grout.
Another apparatus and method for continuously or intermittently advancing tunnel supports against surrounding earth pressure is shown in U.S. Pat. No. 3,613,384. The cutting edge and trailing shells are interconnected by a longitudinal frame or cage structure. Intermediate the forward and trailing shell are overlapping intermediate shells connected individually to the cage structure by hydraulic cylinders so that each intermediate shell can be moved longitudinally relative to the others and relative to the tunnel wall while the other intermediate shells engage the tunnel wall and advance the cage as well as the forward or support shell and the trailing shell. The intermediate shells are moved forward sequentially by releasing pressure exerted against the wall, as by contracting the shell.
Where the tunnelled ground is clay, or other loose soil lacking in consistency, conventional prior art methods involve manual or mechanical excavation, shoring and installation of temporary supports or centers, then driving forward, consolidating the exposed walls, and ultimately casting the tunnel lining.
In this type of procedure, consolidation consists generally in driving piles into and jet grouting the entire supporting wall of the tunnel; needless to say, the piles remain embedded, and will be concealed behind the lining of the tunnel once in place.
Methods of the kind in question are also beset by certain limitations and drawbacks, namely:
high costs deriving from the slow rate of progress and the high manning requirement which accompanies the various steps of the procedure; PA1 dangerous operating conditions (risk of collapse at the workings), for those occupied in excavation and erecting temporary centerings; PA1 waste of materials produced in erecting temporary structures pending installation of final linings.
The object of the present invention is to permit of excavating a tunnel of any given diameter or cross section, and of whatever length, without subjecting pre-cast linings to high thrust stresses.
A further object of the invention is to achieve a considerable reduction in the cost of shoring up the tunnel walls during the course of excavation. Another object of the invention is to provide a temporary shield structure during excavation and subsequent casting/lining works such as will ensure maximum safety during the construction of tunnels of any given size and length, undertaken in loose or unstable ground.
Yet another object of the invention is to enable excavation of the tunnel using conventional and readily available digging equipment, thus bringing the advantages of low running costs and the option of varying excavation and spoil-removal methods in such a way as will best adapt to the type of ground encountered in the course of tunnelling.
An additional object of the present invention is to render the steps of excavation, driving and lining independent of one another.